Diminishing petroleum resources have made the search for alternative fuel and chemical feedstock sources increasingly important. Butanol, in addition to its many uses as a chemical feedstock, is among the alternatives. The acetone/butanol/ethanol (ABE) fermentation, therefore, has received considerable attention in recent years as a prospective process for production of commodity chemicals from biomass..sup.3,6
ABE fermentations are biphasic. During the first (acidogenic) phase logarithmic growth is accompanied by acetic and butyric acid production which also causes a drop in pH. In the second (solventogenic) phase growth ceases and the solvents (ABE) are produced concomitant with consumption of already produced acids and further consumption of the carbohydrates. Hydrogen and carbon dioxide are produced throughout the fermentation.
Traditionally, the commercial ABE fermentation was conducted only in a batch mode due to culture instability and spore-forming nature of the organism. Several solvent-yielding fermentation processes using batch or continuous cultures.sup.2,4,7, chemostats with cell recycling.sup.1 or immobilized cell systems.sup.5 have been described. These processes yield butanol, acetone and ethanol in a ratio of 6:3:1.sup.11. Mixed solvent yields of 29-33% of fermentable carbohydrate have been reported in the literature..sup.12 A total solvent concentration of about 16-20 g/L and a butanol concentration of 10-12 g/L is generally the limit due to toxicity of the butanol produced..sup.10
When C. acetobutylicum is grown in a chemostat, different proportions of acids and solvents are produced depending on the dilution rate and the medium composition. In batch fermentation with the spore-forming strain, selectivity and stability is affected by high carbohydrate concentrations and for this reason high carbohydrate containing fermentations have not been practiced.
For a fermentation process to produce butanol and solvents from carbohydrates to be economical, the solvent yield, concentrations and productivity should be as high as possible.
To make the ABE fermentation economically viable, a number of problems must be addressed. The first of these relates to product toxicity. C. acetobutylicum is intolerant to high concentrations of butanol,.sup.8 with, as little as 1.3% inhibiting growth and fermentation. However, it is important to note that an increase in the butanol concentration from 1.2% to 2% in the fermentation broth would halve the energy consumption for distillation..sup.9 The second problem relates to the fermentation of a high level of initial substrate concentration. C. acetobutylicum ATCC 4259, will not grow and ferment an initial substrate level higher than 78-80 g/L. Thus, achievable levels of butanol and solvents are limited. An equally important problem with a C. acetobutylicum culture is its sporulation which as mentioned earlier is associated with inefficiency of the culture in terms of solvent production. Another problem relates to low productivity. The productivity of the ABE process could be increased by enhancing the fermentation rate of the culture. The organism used for fermentation should have high butyrate uptake activity so that the intermediary compound ends up in butanol and no residual butyrate is left in the fermentation broth. Finally, the instability of the conventional ABE fermentation process is another problem.